Thermoplastic resins exhibit excellent physical properties such as low specific gravity, good moldability and good impact resistance, as compared with glass or metal. With the trend of low cost, larger and lighter weight electric/electronic products, plastic products made of thermoplastic resins are quickly replacing existing glass or metal-based products, thereby broadening applications thereof from electric and electronic products to automobile components.
Particularly, demand for transparent resins has increased in line with recent trend of reduction in thickness of electric/electronic products and variation of design concept. As a result, there is increasing demand for functional transparent materials prepared by imparting functionality such as scratch resistance or flame retardancy to existing transparent resins.
One example of transparent scratch resistant resins is an acrylic resin represented by polymethyl methacrylate (PMMA). Particularly, PMMA exhibits excellent properties in terms of transparency, weather resistance, mechanical strength and scratch resistance, but has low impact resistance and flame retardancy. To enhance impact resistance while maintaining transparency of PMMA, acrylic impact modifiers prepared to have the same index of refraction as that of PMMA are generally used. However, the acrylic impact modifiers have a drawback of lower impact resistance efficiency than butadiene-based impact modifiers. In addition, the method of adding a flame retardant to secure flame retardancy of PMMA does not secure flame retardancy, and can reduce other properties such as thermal resistance and impact resistance while deteriorating thermal stability due to the flame retardant upon processing. As a result, there is no report up to now that the transparent acrylic resin is used alone to provide flame retardancy.
In addition to PMMA, a transparent ABS resin can be used as a transparent scratch resistant material. A methyl methacrylate-acrylonitrile-butadiene-styrene copolymer resin (hereinafter, “transparent ABS resin”) is used to enhance transparency of existing ABS resins. The transparent ABS resin is prepared by blending a methyl methacrylate-styrene-acrylonitrile copolymer resin (hereinafter “MSAN resin”) with a graft copolymer (hereinafter “g-MABS resin”), which is prepared by graft polymerization of styrene monomer and acrylonitrile monomer together with methyl methacrylate monomer to a butadiene-based rubbery polymer. Here, the MSAN resin exhibits good properties in terms of transparency and scratch resistance while providing good mechanical properties, such as impact resistance and the like, as compared with PMMA. In addition, the MSAN resin has high refractivity and thus exhibits high transparency with the butadiene-based impact modifier. However, like the PMMA resin, the transparent ABS resin prepared using the MSAN resin does not secure good flame retardancy due to the methyl methacrylate having low flame retardancy only by addition of the flame retardant, and can suffer from deterioration in impact resistance, thermal resistance, and thermal stability upon processing due to the flame retardant.
In the present invention, a phosphorus (meth)acrylic monomer is introduced to prepare a phosphorus-based copolymer resin, which exhibits excellent flame retardancy without using a flame retardant and has high refractivity while maintaining excellent transparency and scratch resistance.